The present invention relates to metal deposition on semiconductor substrates in semiconductor processing systems, and in particular to the elimination of residue remaining after the etching of the metal.
One of the steps in the formation of a semiconductor chip is the creation of metal interconnections between devices on a semiconductor wafer. Typically, this is done by first depositing a layer of metal, such as tungsten, across the wafer. In one process, tungsten is deposited in a chemical vapor deposition (CVD) chamber by reacting WF.sub.6 containing the tungsten and H.sub.2 or Silane at elevated temperatures over a wafer sitting on a resistive heater (alternately, other methods may be used to heat the wafer, such as lamps). The process temperature is typically 475.degree. C. Subsequently, the tungsten is etched away except for areas in which metal interconnections are desired. This may be done in the same chamber or by moving the wafer to a separate chamber.
It has been discovered that under certain processing conditions, when the wafer is later etched, tungsten residue remains on the wafer, which can form shorts in the interconnects. Although the process is not completely understood, it is believed that tungsten may preferentially grow in certain spots on the wafer (These spots may be areas where residue has formed as a result of previous processing steps). The preferentially growing tungsten may be in a different phase than other areas of the metal tungsten layer. Because the tungsten is in a different phase, these areas may then etch away at a different rate, leaving the undesirable tungsten residue in areas where there should be no interconnect after the etch step.
One approach to eliminate the residue is to clean the chamber in which the metal layer is deposited after every wafer is processed. This is typically done by removing the wafer, and then igniting a plasma composed of NF.sub.3, followed by a hydrogen plasma, with a subsequent argon gas purge. After this cleaning step, the next wafer can be inserted into the chamber for depositing of the tungsten layer without significant residue being present after etchback. A drawback of this plasma clean approach is that it requires a significant amount of time between each wafer being processed. One approach would be to do the plasma less often. But it has been discovered, for example, that a plasma clean after every 25 wafers is not sufficient since residue formation still occurs.
Accordingly, it would be desirable to have an improved process for eliminating residue formation after tungsten deposition and etchback which does not significantly impact wafer throughput.